Some packing machines are equipped with at least partly metal belts, which are heated by electromagnetic induction to seal wrapping material. On machines of this sort, each belt forms part of a sealing unit, which, in addition to the belt looped about two pulleys, comprises a device for detecting and controlling the temperature of the belt.
The belt is fed along a given path and defines a work branch along which it is brought into contact with a sheet of thermoplastic wrapping material, wrapped about an article, to melt a portion of the wrapping material and seal the sheet of wrapping material about the article.
The temperature detecting and control device comprises a ferromagnetic core connected to the belt and to a primary supply circuit to induce a current in, and heat, the belt by induction; and a sensor for detecting the temperature of the belt. The detected temperature is used to control the supply circuit and keep the temperature of the belt around a set value during operation of the packing machine.
Since the belt travels continuously at high speed, the temperature detecting sensors are noncontacting types, normally pyrometers or infrared thermocouples, which have the disadvantage of being not only expensive but also fairly inaccurate. Such sensors, in fact, are based on spectral analysis of the emission of the belt, which changes color uncontrollably during use, thus affecting emission and impairing the accuracy of the temperature measurement.